SAN FRANCISCO, May 1 (Reuters) - The California State Senate advanced a bill on Thursday to restrict the use of antibiotics in farm animals for growth enhancement by requiring that the drugs be sold by prescription for medical reasons only, officials said.

The first-in-the-nation legislation would codify into law voluntary U.S. Food and Drug Administration guidelines, issued late last year, aimed at stemming a surge in resistance to certain antibiotics in humans, according to state Senator Jerry Hill, the bill's author.

"The more antibiotics are used, the more resistance will develop," Hill, a Democrat, said in a statement. "This is an emergent public health issue."

Antibiotic resistance, which can cause humans to lose the ability to fight infections, is thought to be caused partly by the prevalence of the drugs in animal products. Some 70 percent of antibiotics used by humans are also administered to farm animals, Hill said, citing an independent analysis of FDA data.

A recent report by the Centers for Disease Control and Prevention said at least 2 million people are infected annually with antibiotic-resistant bacteria. Some 23,000 people die "as a direct result of the infections," the report said.

Hill's legislation would require that antibiotics be sold with a veterinarian prescription and that antibiotic manufacturers label the drugs to show that they require prescriptions.

But the measure would apply only to antibiotics used by humans, including tetracycline and penicillin, and would not restrict drugs used solely for animals.

Current state law does not restrict the use of antibiotics to produce bigger, fatter animals or require veterinarian prescriptions for the drugs to be sold, Hill's office said.

"The livestock farmers and ranchers want to make sure antibiotics remain effective," said Noelle Cremers, director of Natural Resources and Commodities at the California Farm Bureau Federation.

Passed by a 34-1 senate vote, the measure will now head to the state assembly for another round of votes. If approved, California would be the first state to ban human-used antibiotics on farm animals without a prescription, Hill's office said. (Editing by Cynthia Johnston and Richard Chang)